mass spectrometric monitoring of interfacial photoelectron transfer and imaging of active crystalline facets of semiconductors

We describe herein a mass spectrometric approach to investigate the ultrafast transfer of photoelectrons that are generated by ultraviolet irradiation on surfaces of semiconductor nanopa

Mass spectrometric monitoring of interfacial

photoelectron transfer and imaging of active

crystalline facets of semiconductors

Hongying Zhong1, Juan Zhang1, Xuemei Tang1, Wenyang Zhang1, Ruowei Jiang1, Rui Li1, Disong Chen1,

Peng Wang1 & Zhiwei Yuan1

Monitoring of interfacial electron transfer (ET) in situ is important to understand the ET

mechanism and designing efficient photocatalysts We describe herein a mass spectrometric

approach to investigate the ultrafast transfer of photoelectrons that are generated by

ultraviolet irradiation on surfaces of semiconductor nanoparticles or crystalline facets The

mass spectrometric approach can not only untargetedly detect various intermediates but

also monitor their reactivity through associative or dissociative photoelectron capture

dissociation, as well as electron detachment dissociation of adsorbed molecules

Proton-coupled electron transfer and proton-uncoupled electron transfer with radical initiated

polymerization or hydroxyl radical abstraction have been unambiguously demonstrated

with the mass spectrometric approach Active crystalline facets of titanium dioxide for

photocatalytic degradation of juglone and organochlorine dichlorodiphenyltrichloroethane

are visualized with mass spectrometry imaging based on ion scanning and spectral

reconstruction This work provides a new technique for studying photo-electric properties of

various materials

1 Mass Spectrometry Center for Structural Identification of Biological Molecules and Precision Medicine, Institute of Public Health and Molecular Medicine Analysis, Key Laboratory of Pesticides and Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University, Wuhan, Hubei

430079, China Correspondence and requests for materials should be addressed to H.Z (email: hyzhong@mail.ccnu.edu.cn).

Photo-induced heterogeneous electron transfer (ET) across

the interface between semiconductors and adsorbed

molecules has been involved in various photocatalytic

reactions1–3 and applications to solar energy conversion4–6 or

environmental cleanup7–9 Because of the rapid recombination of

photo generated electrons and holes, it still remains challenging

to study the mechanisms of ultrafast electron transfer processes

with current techniques10–12 Ideally, those techniques not only

should have the capabilities for charge trapping but also should be

able to detect and identify various known/unknown resultant

species, short lived intermediates, as well as location of active

crystalline facets with high-spatial resolution Microscopic

fluorescence imaging based on organic dye probes has recently

been developed as an emerging approach for sensitive detection

of resultant reactive oxygen species and the visualization of active

facets on individual semiconductor particles13–15 In this

technique, organic dye probes are usually designed to have two

parts: a fluorogenic chromophore for microscopic detection and a

reactive site for linking with known or predicted products of

photocatalytic reactions Other unknown or unpredicted species

that cannot be recognized by the probe will not be detected

Because of the structural versatility of intermediates and products,

this targeted technique cannot provide detailed structural

and electronic information on the overall photocatalytic

reactions In addition, to understand the formation mechanisms

of intermediates and final products, energies needed for chemical

bond cleavages are required but they are not available by using

fluorescence imaging techniques alone

In this work, a mass spectrometric approach was designed to

untargetedly detect products along with intermediates of

photo-catalytic reactions and visualize active crystalline facets Recently,

we have reported a new ionization method based on interfacial

photoelectron transfer for mass spectrometric imaging16 This

work is further aimed at facet-dependent photocatalytic activities

of semiconductor crystalline materials, as well as interactions of

photoelectrons with adsorbed molecules Advantages of mass

spectrometers for such studies include the high-vacuum sample

chamber and the built-in static electric field with laser pulses

The high-vacuum chamber eliminates interferences of abundant

atmospheric N2 and O2, as well as solvents It also simplifies

theoretical calculations that have been used for evaluation of

bond energy, ion stability, reaction thermodynamics and kinetics

The built-in electric field facilitates instant separation of

photo-generated electron–hole pairs as soon as the laser beam irradiates

on surfaces of semiconductors Once photoelectrons are captured

by adsorbed molecules, resultant negatively charged radical

anions are pulled out of the surfaces of semiconductor

nanoparticles in the static electric field Therefore various

intermediates that are under-revealed previously may be

detected with mass spectrometers In addition, because kinetic

energies of photoelectrons are controllable through adjusting the

bias voltage between the sample plate and the aperture, it is very

convenient to monitor the energies needed for chemical bond

cleavage Compared with other existing spectroscopic methods

such as fluorescence or electron spin resonance, the unique

feature of mass spectrometry is the capability to provide

information on both masses and charges that are essential for

structural interpretation of intermediates and final products

In this work, the proposed mass spectrometric approach has

been applied to investigate photocatalytic reactions of juglone and

organochlorine 4, 40-dichlorodiphenyltrichloroethane (DDT) on

surfaces of semiconductor nanoparticles Different intermediates

and product ions have been found through associative or

dissociative photoelectron capture dissociation, as well as electron

detachment dissociation In addition to well recognized

proton-coupled electron transfer, radical initiated polymerization or

hydroxyl radical abstraction have also been unambiguously demonstrated with the mass spectrometric approach In contrast

to optical microscopy which shows the physical shapes, mass spectrometric imaging reveals spatial distribution of ions Active crystalline facets of titanium dioxide have been visualized by scanning all know and unknown degradation products or intermediate ions It is shown that this mass spectrometric approach should be able to provide a new way for exploring photo-electric properties of various materials

Results Mass spectrometric monitoring of photoelectron transfer Compared with the general setup of a microscopic fluorescence imaging approach shown in Fig 1a, by which only predicted species are detected, the mass spectrometry-based approach takes the full scan manner and all ions except radicals and neutral species are detected As shown in Fig 1b, an air dried titanium dioxide crystal (rutile) with exposedo1004 facet that has been soaked in the solution of electron acceptor juglone was stuck to a conductive alumina tape before the assembly was fixed to the sample plate In the quadrupole time-of-flight mass spectrometer, ultraviolet laser pulses are synchronized with the detector While the laser beam (355 nm) scans across the faceto1004 point by point, photoelectrons are instantly captured by adsorbed electron acceptor molecules (black balls) and photo-generated ions (coloured balls) are recorded by the detector Mass spectra are then reconstructed to image site-specific photocatalytic reactions This approach can also be used for studies of photoelectron transfer from bulk semiconductor nanoparticles In such case,

a suspension of nanoparticles in isopropanol was pipetted onto the sample well After air-dried, the sample plate was then subjected to ultraviolet irradiation

Juglone and DDT have been chosen for proof-of-principle demonstration Because the wavelength of the laser used in this work is 355 nm, we have first checked the ultraviolet absorption spectra of these two compounds in order to ensure that their ionization is not due to direct ultraviolet absorption Supplementary Figure 1 shows both of them do not have strong absorption at 355 nm In addition, production of photoelectrons, when light is shone onto a material has been well known To confirm that the ionization of juglone and DDT surely result from the capture of photoelectrons, insulated glue tapes have been put

on the surface of the sample plate to block the interfacial transfer

of photoelectrons, as shown in Supplementary Fig 2 It was found that no signal can be obtained Without the use of semiconductors, direct irradiation of ultraviolet light on juglone

or DDT cannot cause the formation of ions that can be detected

by the mass spectrometer

Associative trapping and detection of photoelectrons So far, photoelectrons or holes generated on surfaces of semiconductors have rarely been directly observed with current techniques Photoelectron spectroscopy is one of the alternatives but it does not provide information on interfacial interactions of photoelectrons with adsorbed molecules17,18 Although these interactions can be monitored by chemical reactions of fluorescence dye probes with resultant reactive oxygen species

or hydroxyl radical19–21, experimental evidences are needed for further validation because how and why reactive oxygen species

or hydroxyl radicals are produced upon light irradiation remain unknown The strategy described herein is to use an electron acceptor such as juglone as the molecular probe to trap photoelectrons In the absence of O2, capture of photoelectrons switches neutral juglone molecules to negatively charged radical anions that can be detected in the negative ion mode of the mass

c

d

e

Charge distribution at different carbon atoms:

C3 a b c d

f e

a b

c

f e g

d g

C1

C3

C2

C1

Neutral juglone

H

–

–

b: –0.226 c: –0.103 d: –0.160 e: –0.291 f: –0.208 g: –0.205

Excited state

Radical anion of juglone

Reaction coordinate

Conduction band

Valence band

+

Electrons

Holes

IC

Photoexcited state

Ground state Antibonding orbital

Π*

Π*

Π

Π

3 ns

Pulse Laser

a: –0.303 b: –0.299 c: –0.098 d: –0.162 e: –0.353 f: –0.248 g: –0.239

Juglone with an electron C2

V2 V3

m/z

m/z 174.0317

Bias voltage

<100>

<100>

<100>

Orbital – – – – – –

Figure 1 | Experimental design and theoretical investigation of interfacial photoelectron transfer (a) Principle of fluorescence spectroscopy-based approach (b) Principle of mass spectrometry-based approach (c) Charge distribution of neutral juglone and the radical anion of juglone (d) Energy profile

of photoelectron capture by neutral juglone (e) Formation of anionic ground states through ultrafast internal conversion Purple ovals: fluorogenic chromophores Black balls: molecules adsorbed on different sites of semiconductor facets Coloured balls (red, green, orange and blue balls): resultant intermediates and products of potocatalytic reactions V1, V2 and V3: the voltages applied to the extraction plate, hexapole and aperture, respectively.

spectrometer Once radical anions are formed, they are instantly

pulled out of the surfaces of semiconductors in the static electric

field Detection of radical anions provides direct experimental

evidence on the interfacial electron transfer and photoelectron

capture As shown in Fig 1c, calculations with density functional

theory (DFT) indicate that there are three charge deficient carbon

atoms labelled as C1, C2 and C3 respectively in the neutral

juglone These carbon atoms have possibilities to capture

photoelectrons With an acquired electron, the carbon atom

labelled as e of the juglone becomes the most negatively charged

carbon atom which implicates the preferable electron capture at

the carbon atom labelled as C1 Actually, it can be found that the

acquired negative charges and unpaired electrons present in

resultant radical anions are delocalized over the whole ions, when

photoelectrons are captured at this position Delocalization of

charges stabilizes the radical anion and promotes the proceeding

of such interfacial charge transfer In contrast, although carbon

atoms labelled as C2 and C3 are also charge deficient, acquired

charges by these two carbon atoms cannot be as well delocalized

as that of the carbon atom labelled as C1 The energy profile of

the photoelectron trapping process is shown in Fig 1d Changes

of free energies and enthalpies after neutral juglone traps

low energy photoelectrons are  35.8 and  35.6 kcal mol 1,

respectively Therefore photoelectron trapping by juglone is an

exothermic process that can spontaneously proceed It has been

theoretically demonstrated that trapping of photoelectrons by

juglone molecules is energetically favourable

As we know, radical anions have been considered as highly

reactive species22–24 The time for an ion generated in the sample

chamber to be detected by a quadrupole time-of-flight mass

analyser is usually about in the scale of microsecond Detection of

such intermediate radical anions of juglone implicates an ultrafast

relaxation mechanism that can account for the ability of juglone

to trap and retain photoelectrons By using a combination of

time-resolved photoelectron spectroscopy and high level ab initio

calculation, Verlet et al has demonstrated that formation

of anionic ground states through ultrafast internal conversion

attributes to the stability of radical anions of para-benzoquinone,

which is a derivative of juglone25 As shown in Fig 1e, internal

conversion would re-distribute excess internal energy among all

vibrational modes that can be quenched by the surroundings

Because internal conversion process is in the timescale of

sub-40 fs, which is much faster than that of detection time

(Bms), it is feasible to observe anionic radicals in stable ground

state So it has been theoretically demonstrated that the detection

of intermediate radical anions resulting from the trapping of

photoelectrons is also energetically favourable

The formation of an intermediate radical anion was further

experimentally confirmed by the dominant molecular ion of

juglone at m/z 174.0324 Da that was generated on surfaces of zinc

oxide nanoparticles It was observed in the negative ion mode of

the mass spectrometer with 0.1 V of bias voltage Setting of such

low bias voltage is aimed to avoid vibrational excitation that may

cause break down of ions The ion at m/z 174.0324 Da in Fig 2a

has the same mass as that of the neutral molecule (error:

0.0007 Da) As we know, in the negative ion mode of the mass

spectrometer, only negative ions but not neutral molecules can be

detected Observation of the ion with the same mass as that of the

neutral molecule unambiguously indicates the capture of

photoelectrons To further validate that a photoelectron was

indeed captured by the neutral molecule, the same sample has

been analysed in the positive ion mode As shown in Fig 2b, an

ion at m/z 176.0531 Da was observed in addition to usual

protonated ion at m/z 175.0441 Da The formation mechanism of

these ions was illustrated in Fig 2c Compared with the mass of a

neutral molecule, the 1.0090 Da mass differences represent the

addition of one more proton (error: 0.0012 Da) Because the OH groups of juglone molecules have weak acidity, juglone molecules can generate protons by themselves through de-protonation mechanism with laser irradiation With two proton mechanism shown in Fig 2c, the first proton neutralizes a radical anion formed through photoelectron capture and then the second proton causes re-ionization of the neutralized molecule The protonation process was thought to occur at the oxygen atoms of two carbonyl groups of juglone because of the known basicity of carbonyl groups in organic chemistry26 In addition, because negative charges carried on radical anions are delocalized over the ions, the carbon anion does not have basicity and it should not be protonated However, electrostatic interactions between positive protons and radical anions may facilitate the protonation process Supplementary Fig 3 shows that the two proton mechanism was not applicable to baicalein because it has only one carbonyl group Similar results have been obtained on surfaces of anatase titanium dioxide nanoparticles (Supplementary Fig 4) These experimental results demonstrate the proton-coupled interfacial photoelectron transfer and the production of radical anions with laser irradiation on semiconductor nanoparticles

Dissociative trapping and reactivity of photoelectrons Because

of the presence of very active unpaired electrons, intermediate radical anions undergo subsequent chemical bond cleavage and new bond formation Reactivity of intermediate radical anions is monitored by in situ mass spectrometric detection of secondary intermediates or products When the bias voltage between the sample plate and the aperture increases, kinetic energy of photoelectrons gradually increases which allows sequential cleavage of different bonds with different bond energies When kinetic energy of photoelectrons is controlled as 20 eV or even lower, the de Broglie wavelength of photoelectrons does not match general bond length of organic molecules Under such condition, photoelectrons can only be exothermally captured by charge deficient atoms and initiate electron-directed chemical bond cleavage without vibrational activation Nanoparticles of zinc oxide have been used for experimental demonstration of the reactivity of intermediate radical anions In Fig 3a, the radical anion at m/z 174.031 Da is dominant at 20 V bias voltage The ion

at m/z 173.0233 Da has 1.0081 Da mass shifts than the radical anion at m/z 174.0314 Da The mass differences indicate the loss

of an H atom (error: 0.0003 Da) When the bias voltage increases from 0 to 60 V, relative intensities of the ion at m/z 174.0314 Da decreases from 100 toB20%, while the ion at m/z 173.0233 Da increases from B20 to 100%, as shown in Fig 3a–c Overall intensity trends of these two ions are shown in Fig 3d and e, respectively There are three interesting findings from Fig 3 (1) Loss of an H atom is preferred because of the lowest bond energy and the stability of resultant ions The ion at m/z 173.0233 Da results from the loss of an H atom It was observed even with only 0.1 V bias voltage because photons of ultraviolet irradiation can provide enough energy for O–H bond cleavage Delocalization of negative charges over the aromatic ring of resultant ions accounts for the resonance stabilization (2) Dissociation of radical anions rapidly rises, when the bias voltage was increased to 420 V Increased bias voltage results in the formation of two ions at 145.0303 Da and 117.0332 Da with 27.9929 Da and 27.9972 Da mass shifts to the radical anion respectively, which represents the losses of CO (error: 0.0020 Da and 0.0023 Da, respectively) through sequential cyclocondensa-tion reaccyclocondensa-tions The ion at m/z 145.0303 Da was observed along with the ion at m/z 173.0233 Da when the bias voltage was set as

20 V When the bias voltage was increased to 30 V, the relative intensity of the ion at m/z 145.0303 Da approached B70%

The ion at m/z 117.0332 Da was not observed until the bias

voltage was increased to 460 V These experimental results

indicate that the cyclocondensation reaction needs much more

energies than that to break down O–H bonds (3) When Fig 3d

was compared with Fig 3e, it was found that there are larger

intensity deviations for the ion at m/z 173 Da than that of the

ion at m/z 174 Da, which is in accordance with the internal

conversion process demonstrated in Fig 1e Because of the

internal conversion process, a series of low-lying electronic states

with different energies of the radical anion at m/z 174 Da results

in various degrees of degradation

Degradation mechanisms are illustrated in Fig 4a It is shown

that fragment ions can be generated by either specific a cleavage

of O–H bond or sequential losses of CO molecules through

cyclocondensation However, Fig 3c shows that even with 60 bias

voltages, the ion at m/z 89.0391 still could not be detected through photoelectron capture dissociation Instead, as shown in Fig 4b, it was detected with argon collision activated dissociation DFT calculation summarized in Fig 4c indicates that changes in free energies for all these degradation reactions are 40, meaning these degradations cannot proceed spontaneously without additional energies In fact, there are much larger DG values for ions at m/z 117.0332 Da and 89.0391 Da than that of other ions

It is in accordance with experimentally observed much higher energies needed for those degradations

It is now clear that capture of photoelectrons results in the formation of stable radical anions over which acquired electrons are delocalized We have also demonstrated that this mass spectrometric result is in accordance with what we observed in real atmospheric condition Supplementary Figure 5 shows the

100

a

b

c

Negative ion mode

Positive ion mode

~ 0.1 V

174.0324

173.0246

~ 0.1 V

176.0531

175.0441 160.0548

%

%

80 60

40 20

0

100 80 60

40

20 0

60

Ö

Ö

OH

Ö

H +

Ö

H +

Ö

H +

H + OH

–

OH

m/z

m/z

m/z 175.0395

m/z 176.0473

Protonation

Two proton mechanism

Figure 2 | Illustration of associative photoelectron capture on nanoparticles of zinc oxide (a) Mass spectrum of juglone in the negative ion mode (b) Mass spectrum of juglone in the positive ion mode (c) Mechanisms of one-step protonation and two-step protonation The bias voltage was set as B0.1 V for all these experiments.

pictures for the original juglone solution and the solutions mixed

with titanium dioxide nanoparticles under ultraviolet irradiation

alone or together with 100 °C heating, respectively It was found

that ultraviolet irradiation alone did not cause the degradation of

juglone molecules unless additional energies were provided To

confirm the degradation mechanism shown in Fig 4a,13C NMR

technique has been applied to monitor changes in carbon atoms

Supplementary Figure 6a–c shows that all carbon peaks of

carbonyl groups labelled as 1, 2 and 3 were still detected even

under ultraviolet irradiation for 5 h Altogether with ultraviolet irradiation and heating, these peaks disappeared The disappeared carbon peaks represents the sequential losses of CO molecules Related1H NMR spectra are shown in Supplementary Fig 6d–f This NMR observation is in accordance with what we have shown

in the mass spectra of Figs 2 and 3 It also validates the proposed mechanism shown in Fig 4a In the mass spectrometer, only when the bias voltage was increased to 420 V, degradation of juglone can be detected Similar results have also been observed in

100

a

b

c

20 V

30 V

60 V

174.0314

173.0233

174.0285 173.0206

145.0303

145.0304

117.0332

173.0233

174.0303

m/z

m/z

m/z

%

80 60 40 20 0

100

%

80 60 40 20 0

100

%

80 60 40

120 100 80 60 40 20 0

120 100 80 60 40 20 0

Bias voltage (V)

Bias voltage (V)

20 0

Figure 3 | Dissociative photoelectron capture on nanoparticles of zinc oxide under different bias voltages Mass spectra of juglone were acquired

in the negative ion mode under different bias voltages (a) 20 V (b) 30 V (c) 60 V (d) Overall intensity trend of the negative ion at m/z 174 Da and (e) 173 Da under different bias voltages.

the solution of juglone suspended with semiconductor

nanoparticles of zinc oxide that has been subjected to ultraviolet

irradiation in atmospheric condition (Supplementary Fig 7)

With increased energies, intensities of fragment ions with losses

of CO molecules increase In summary, the reactivity of

intermediate radical anions is due to unpaired electrons

Stabilization of acquired charges and energies needed for bond

cleavage play important roles in unpaired electron initiated

reactions

Radical initiated polymerization reactions In contrast to

dis-sociation, it has been observed that highly reactive radicals can

actually also initiate polymerization reactions, which provides another experimental evidence for the occurrence of interfacial photoelectron transfer and the presence of unpaired electrons in resultant species Figure 5a indicates that there are several ions with m/z values much higher that of the radical anion and fragments (labelled as red stars) As shown in Fig 5b, the highly active unpaired electron present in the radical anion can activate the two a-positioned bonds and cause homolytical cleavage of adjacent C–H and O–H bonds Then newly formed radicals react with each other to pair electrons and generate stable dimmers (error: 0.0014 Da), trimmers (error: 0.0001 Da) and tetramers (error: 0.0007 Da) Both open-chain structures and ring-shape structures with 2.0155 Da mass shifts have been observed for

O

a

b

c

+

+

–

–

–

–

H O

O

m/z 174.0317

m/z 145.0290

m/z 89.0391

m/z 117.0340

m/z 173.0239

O

H OH

100

MS/MS with argon collision

Free energy (Hartree)

–610.2637 O

O OH

O O

O

O

O

O OH –610.3208

–382.9051 – –

– –

–

ΔG=71174.8

–496.2991

89.0381

117.0342

145.0305

173.0282

%

80 60 40 20

–620

–570

–520

–470

–420

–370

Reaction coordinate

0

m/z

–

Figure 4 | Reactivity of radical anions in dissociative photoelectron capture (a) Degradation pathways to generate negative ions at m/z 173.0239, 145.0290, 117.0340 and 89.0391 Da, respectively (b) Tandem MS/MS spectrum of juglone with argon collision activated dissociation (c) The free-energy profile along degradation reaction co-ordinate.

those trimmers and tetramers Identities of these polymers are not

only validated by their accurate masses but also further confirmed

by the 13C NMR spectrum (Supplementary Fig 6) In fact,

careful examination of Supplementary Fig 5 reveals that the

colour of the juglone solution became darker under ultraviolet

irradiation In contrast to degradation reactions, this result

indicates that new species with larger molecular absorption

coefficients have been produced when the juglone solution mixed

with titanium dioxide nanoparticles has been subjected to

ultra-violet irradiation Supplementary Figure 8 shows that dimmers,

trimmers and tetramers are still present in the solution even with heating

In particular, it is very interesting that only the open-chain structured dimmer at m/z 346.0491 Da was found but the ring-shape structured dimmer theoretically at m/z 344.0321 Da was not found By looking at Fig 5b, it was recognized that a rigid structure was confined within the aromatic plane Because of the steric effect, the ring-shape structured dimmer cannot be formed and elongation reactions should be terminated by a hole oxidization mechanism

100

a

b

174.0318

346.0491

516.0482 518.0637 688.0649

690.0790 145.0289

%

80 60 40 20 0

O

O H

O

O

O

O

H O O

O

H H

H

O

O O

–

–

–

O

O

O O

O O

O – –

–

O

O O

+

+

O

O OH

O

O

O

O

O O

O O

O O

O

O O O

O

O

O

O

O O

OH

m/z

m/z 346.0477

m/z 174.0317

m/z 518.0638

m/z 516.0481

Figure 5 | Radical initiated polymerization reactions (a) Mass spectrum of the ions with m/z values higher than that of original juglone (b) Pathways for generation of dimmers, trimmers and tetramers of juglone.

Roles of hydroxyl radicals in degradation pathways In addition

to reductive pathways through either associative or dissociative

electron capture and radical initiated polymerization, hole

oxidization generated hydroxyl radicals have also been considered

as an important intermediate for efficient photo degradation of

pollutants27–31 It has been proposed that hydroxyl radicals can

react with other molecules through the abstraction of a hydrogen

atom from a C–H bond32 However, detection of hydroxyl

radicals has been proven difficult because of the rapid

degradation of resultant products In this work, by using the

mass spectrometry, intermediate ions are instantly extracted in

the static electric field and thus can be detected Figure 6a–c

represents the ions generated on surfaces of titanium dioxide

nanoparticles with ultraviolet irradiation at 20 V, 30 V and 60 V

bias voltages, respectively Attention has been attracted to two

ions at m/z 161.0267 Da and 189.0213 Da Judged by accurate

masses, these two ions are produced by the abstraction

of a hydrogen atom from a C–H bond of the ions at m/z

145.0290 Da and 173.0239 Da, respectively (error: 0.0028 Da and

0.0025 Da) They have also been observed in the solution of

juglone that has been subjected to ultraviolet irradiation in real

atmospheric condition (Supplementary Fig 9), as well as the

solution mixed with zinc oxide nanoparticles (Supplementary

Fig 10)

Abstraction of a hydrogen atom from a C–H bond of juglone

by a hydroxyl radical is favourable because the lone pair electrons

of oxygen atom can be delocalized over the aromatic ring, as

shown in Fig 6d Resultant hydrogen atom with an unpaired

electron is also highly reactive Mayer et al has reported

that the addition of stable 2,4,6-tri-tert-butylphenoxyl radical

(tBu3ArO) to air-free toluene solutions (a solvent without acidic

proton) of ZnO/e– or TiO2/e– (amorphous or anatase) can yield

phenol tBu3ArOH (ref 33) A mechanism of proton-coupled

electron transfer (PCET) has proposed for explanation of the

production of tBu3ArOH Although it has been assumed that

active protons likely come from surface hydroxyl groups

originated from particle syntheses, the formation of a hydrogen

atom from hydroxyl group was not explained This work provides

experimental evidence for the presence of hydroxyl radicals

A hydrogen atom resulting from hydroxyl radical abstraction,

instead of a proton, may react with the tBu3ArO radical to

produce phenol tBu3ArOH Because in situ mass spectrometric

detection of gaseous ions was performed in high-vacuum

condition, interferences of solvents are eliminated in this work

It has been demonstrated that the abstraction of hydrogen atoms

with hydroxyl radicals is independent of the acidity of the juglone

solution (Supplementary Fig 11), which is in accordance with

that Mayer et al observed In fact, hydroxyl group have been

widely found on surfaces of different semiconductor

nanoparti-cles such as zinc oxide or titanium dioxide The presence of –OH

has been confirmed with the strong O–H vibrational stretch in

the infrared spectra34 Hole oxidization of –OH group results in

the formation of hydroxyl radicals that can initiate downstream

reactions It has been shown that proton-uncoupled electron

transfer process actually is coupled with the hydroxyl group in

addition to radical initiated polymerization

Intermediates in electron detachment dissociation One of the

important advantages of mass spectrometry is the versatility

to untargetedly detect all ions generated through different

mechanisms When the kinetic energy of photoelectrons was

increased to 440 eV by increasing the bias voltage between the

sample plate and the aperture, ions resulting from electron

detachment dissociation were observed in the positive ion mode

of the mass spectrometer Under low bias voltage such as 20 V,

commonly observed ions that are generated through one-step protonation (m/z 175.0440 Da) and two-step protonation (m/z 176.0516 Da) are shown in Fig 7a When the bias voltage approached 40 V, an ion at m/z 174.0342 Da indicated with a red oval that has the same mass (error: 0.0025 Da) as that of the neutral molecule of juglone is observed in Fig 7b Its intensity increases with increased bias voltage, as shown in Fig 7c This ion was formed because of the loss of an electron DFT calculation shown in Fig 7d, reveals that DG value for electron detachment process is 40 Unlike electron capture dissociation, electron detachment process cannot spontaneously proceed unless additional energy is provided Comparing exothermal photo-electron capture with endothermal photophoto-electron impact, ions are generated through different pathways although degradation reactions are also initiated by unpaired electrons Two protonation of the negative ion at m/z 145.0290 Da detected in negative ion mode of the mass spectrometer results in the formation of the ion at m/z 147.0446 Da detected in positive ion mode of the mass spectrometer Occurrence of such proton-coupled electron transfer processes ascribes to the proton affinity of adsorbed molecules and the energy provided by laser irradiation for O–H bond dissociation

Imaging of active crystalline facets Facet-dependent photo-catalytic performance of semiconductor nanoparticles has been well recognized recently Controlled synthesis of photo active single-crystalline semiconductors with desirable exposed facets needs new techniques that can reveal the activity of each crystalline facet On the basis of the principle of the proposed mass spectrometric approach, active crystalline facets of a single rutile titanium dioxide crystalline have been visualized by scanning the ultraviolet laser across the facet o1004 and adjacent facets In Fig 8a, scanning electron microscope (s.e.m.) has been used to characterize surfaces of semiconductor crystallines It was clearly shown that juglone molecules have been adsorbed on both o1004 and adjacent facets However, very weak signals have been observed on theo1004 facet The stronger photocatalytic activity of the adjacent o1014 facet is proven by the detection of much stronger ions of juglone at m/z 174.0317 Da, 173.0239 Da, 145.0290 Da and 117.0340 Da in Fig 8b Among these ions, the one at m/z 117.0340 Da has the lowest intensity because of higher energy needed to produce this ion In addition, observation of the strong original peak without degradation at m/z 174.0317 provides further experimental evidence on the formation of stable anionic ground states through ultrafast internal conversion as that has been reported By using first-principles calculations, the surface energies of rutileo1004 facet and o1014 facet have been reported as 0.67 J m 2 and 1.01 J m 2, respectively with PAW10 pseudopotential (Projector Augument Wave)35 The much higher surface energy ofo1014 face is in accordance with the experimentally observed higher photocatalytic activity on o1014 facet However, it should be indicated that the crystal-facet dependency of activities is also affected by surface defects, as well as energy levels of the conduction bands

To further demonstrate that photocatalytic activity is not only associated with surface properties, but also highly dependent on properties of adsorbed molecules, organochlorine 4, 40-DDT has been used as an example As shown in Supplementary Fig 12, in contrast to juglone, the original peak of 4, 40-DDT without degradation at m/z 351.9147 Da has much lower intensity than that of the degradation product at m/z 280.9692 Da on the o1014 facet It indicates that capture of the photoelectron by

4, 40-DDT rapidly initiates downstream chemical bond cleavage Because of the electron attracting effect of chlorine atoms, radical

anions produced by photoelectron capture of 4, 40-DDT is not as

stable as that of juglone

Discussion

The mass spectrometry-based approach is capable for monitoring

of interfacial photoelectron transfer and imaging of active

crystalline of semiconductors In the mass spectrometer, recombination of electron–hole pairs is prevented by the

built-in static electric field and built-interferences of atmospheric N2and O2

are eliminated in the intrinsic high vacuum condition Once photoelectrons are captured by adsorbed electron receptors, resultant radical anions are pulled out of surfaces and detected by

100

a

b

c

d

20 V

30 V

60 V

173.0249

174.0313

173.0231

117.0362

m/z 174.0317

m/z 161.0239

m/z 189.0188

145.0294 161.0239 173.0256

174.0336

189.0236

161.0267

189.0213 145.0288

174.0313

%

80

60

40

20

0

100

%

80

60

40

20

0

100

%

80

60

40

20

0

60 O

OH

H –

OH

OH –

H –

m/z

m/z

m/z

Figure 6 | Hydroxyl radical abstraction on nanoparticles of titanium dioxide under different bias voltages (a) 20 V (b) 30 V (c) 60 V (d) Mechanism

of the abstraction of a hydrogen atom of C–H bond by hydroxyl radical.